The number of children with autism disorders surged 57 percent between 2002 and 2006.

The increased prevalence underscores the need to regard autism as an “urgent public health concern,” the report said.

Autism disorders are developmental disabilities characterized by impaired social skills, communication and behavior. There are no medical tests available to diagnose autism. The condition is identified after a child’s developmental progress is evaluated. The cause of autism is unknown. Many researchers believe it may be caused by a combination of genetic and environmental factors.

Between the 1960s and 1980s, autism was believed to affect one in 2,000 children.

Catherine Rice, a behavioral scientist at the CDC, said it’s not clear why prevalence is increasing. She said it may be the result of many factors including: increased awareness of the condition, better documentation of cases in records from schools and health care providers and the possibility that more children at risk.

The children identified in the study represent about 8 percent of U.S. 8-year-olds, so it’s not a nationally representative sample. Rice said the study found very little variation in the prevalence of autism among the states analyzed: Alabama, Arizona, Colorado, Florida, Georgia, Maryland, Missouri, North Carolina, Pennsylvania, South Carolina and Wisconsin.

She said the study’s conclusions about the increased prevalence of autism are very similar to the findings of recent studies done in Europe and Asia.

The CDC study found children are diagnosed, on average, at age 4 ½. Autism disorders are identified in one of every 70 boys and one in every 315 girls.

The updated estimate is particularly important for school districts that provide special education services to children with autism, according to Rice.

“If you are a school system and you are planning for one in 150 children versus one in 110, that’s quite a lot of children,” Rice said. “In this time of budget crunches, it’s really important to have a better estimate of who is getting identified.”